Timing adjustment in multi-hop communication system

ABSTRACT

A timing adjustment method for use in a multi-hop communication system is provided. The method includes transmitting information in a first labelled interval from a first apparatus along a plurality of consecutive links of a path via one or more intermediate apparatuses to a second apparatus, said information including a reference to a particular one of said labelled intervals and the transmission of that information incurring a delay such that the transmitted information, or information derived therefrom, is received by said second apparatus in a second labelled interval a number of such intervals after the first labelled interval. The method also includes adjusting said reference to form an adjusted reference referring to a further labelled interval the or another number of such intervals before or after the particular labelled interval.

RELATED APPLICATION

This application claims foreign priority benefits under 35 U.S.C. §119of International Application No. PCT/GB2007/002887, filed Jul. 31, 2007,entitled “Timing Adjustment in Multi-Hop Communication System” andUnited Kingdom Application No. GB 0616476.8, filed on Aug. 18, 2006,entitled “Communication Systems”.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application relates to the following applications, each of which isincorporated herein by reference:

-   -   COMMUNICATION SYSTEMS, application Ser. No. 11/840,492, filed        Aug. 17, 2007 and currently pending;    -   COMMUNICATION SYSTEMS, application Ser. No. 11/840,518, filed        Aug. 17, 2007 and currently pending;    -   COMMUNICATION SYSTEMS, application Ser. No. 11/840,546, filed        Aug. 17, 2007 and currently pending;    -   COMMUNICATION SYSTEMS, application Ser. No. 11/840,570, filed        Aug. 17, 2007 and currently pending;    -   COMMUNICATION SYSTEMS, application Ser. No. 11/840,595, filed        Aug. 17, 2007 and currently pending;    -   COMMUNICATION SYSTEMS, application Ser. No. 11/840,621, filed        Aug. 17, 2007 and currently pending;    -   COMMUNICATION SYSTEMS, application Ser. No. 11/840,644, filed        Aug. 17, 2007 and currently pending;    -   COMMUNICATION SYSTEMS, application Ser. No. 12/377,629, filed        Feb. 16, 2009 and currently pending;    -   COMMUNICATION SYSTEMS, application Ser. No. 12/377,640, filed        Feb. 16, 2009 and currently pending.

TECHNICAL FIELD

The present disclosure is related to network communication systems, andmore particularly, to timing adjustment in a multi-hop communicationsystem.

BACKGROUND

Currently there exists significant interest in the use of multihoptechniques in packet based radio and other communication systems, whereit is purported that such techniques will enable both extension incoverage range and increase in system capacity (throughout).

In a multi-hop communication system, communication signals are sent in acommunication direction along a communication path (C) from a sourceapparatus to a destination apparatus via one or more intermediateapparatuses. FIG. 19 illustrates a single-cell two-hop wirelesscommunication system comprising a base station BS (known in the contextof 3 G communication systems as “node-B” NB) a relay node RN (also knownas a relay station RS) and a user equipment UE (also known as mobilestation MS). In the case where signals are being transmitted on thedownlink (DL) from a base station to a destination user equipment (UE)via the relay node (RN), the base station comprises the source station(S) and the user equipment comprises the destination station (D). In thecase where communication signals are being transmitted on the uplink(UL) from a user equipment (UE), via the relay node, to the basestation, the user equipment comprises the source station and the basestation comprises the destination station. The relay node is an exampleof an intermediate apparatus (I) and comprises: a receiver, operable toreceive data from the source apparatus; and a transmitter, operable totransmit this data, or a derivative thereof, to the destinationapparatus.

Simple analogue repeaters or digital repeaters have been used as relaysto improve or provide coverage in dead spots. They can either operate ina different transmission frequency band from the source station toprevent interference between the source transmission and the repeatertransmission, or they can operate at a time when there is notransmission from the source station.

FIG. 20 illustrates a number of applications for relay stations. Forfixed infrastructure, the coverage provided by a relay station may be“in-fill” to allow access to the communication network for mobilestations which may otherwise be in the shadow of other objects orotherwise unable to receive a signal of sufficient strength from thebase station despite being within the normal range of the base station.“Range extension” is also shown, in which a relay station allows accesswhen a mobile station is outside the normal data transmission range of abase station. One example of in-fill shown at the top right of FIG. 20is positioning of a nomadic relay station to allow penetration ofcoverage within a building that could be above, at, or below groundlevel.

Other applications are nomadic relay stations which are brought intoeffect for temporary cover, providing access during events oremergencies/disasters. A final application shown in the bottom right ofFIG. 20 provides access to a network using a relay positioned on avehicle.

Relays may also be used in conjunction with advanced transmissiontechniques to enhance gain of the communications system as explainedbelow.

It is known that the occurrence of propagation loss, or “pathloss”, dueto the scattering or absorption of a radio communication as it travelsthrough space, causes the strength of a signal to diminish. Factorswhich influence the pathloss between a transmitter and a receiverinclude: transmitter antenna height, receiver antenna height, carrierfrequency, clutter type (urban, sub-urban, rural), details of morphologysuch as height, density, separation, terrain type (hilly, flat). Thepathloss L (dB) between a transmitter and a receiver can be modelled by:L=b+10n log d  (A)Where d (meters) is the transmitter-receiver separation, b(db) and n arethe pathloss parameters and the absolute pathloss is given byl=10^((L/10)).

The sum of the absolute path losses experienced over the indirect linkSI+ID may be less than the pathloss experienced over the direct link SD.In other words it is possible for:L(SI)+L(ID)<L(SD)  (B)

Splitting a single transmission link into two shorter transmissionsegments therefore exploits the non-linear relationship between pathlossverses distance. From a simple theoretical analysis of the pathlossusing equation (A), it can be appreciated that a reduction in theoverall pathloss (and therefore an improvement, or gain, in signalstrength and thus data throughput) can be achieved if a signal is sentfrom a source apparatus to a destination apparatus via an intermediateapparatus (e.g. relay node), rather than being sent directly from thesource apparatus to the destination apparatus. If implementedappropriately, multi-hop communication systems can allow for a reductionin the transmit power of transmitters which facilitate wirelesstransmissions, leading to a reduction in interference levels as well asdecreasing exposure to electromagnetic emissions. Alternatively, thereduction in overall pathloss can be exploited to improve the receivedsignal quality at the receiver without an increase in the overallradiated transmission power required to convey the signal.

Multi-hop systems are suitable for use with multi-carrier transmission.In a multi-carrier transmission system, such as FDM (frequency divisionmultiplex), OFDM (orthogonal frequency division multiplex) or DMT(discrete multi-tone), a single data stream is modulated onto N parallelsub-carriers, each sub-carrier signal having its own frequency range.This allows the total bandwidth (i.e. the amount of data to be sent in agiven time interval) to be divided over a plurality of sub-carriersthereby increasing the duration of each data symbol. Since eachsub-carrier has a lower information rate, multi-carrier systems benefitfrom enhanced immunity to channel induced distortion compared withsingle carrier systems. This is made possible by ensuring that thetransmission rate and hence bandwidth of each subcarrier is less thanthe coherence bandwidth of the channel. As a result, the channeldistortion experienced on a signal subcarrier is frequency independentand can hence be corrected by a simple phase and amplitude correctionfactor. Thus the channel distortion correction entity within amulticarrier receiver can be of significantly lower complexity of itscounterpart within a single carrier receiver when the system bandwidthis in excess of the coherence bandwidth of the channel.

Orthogonal frequency division multiplexing (OFDM) is a modulationtechnique that is based on FDM. An OFDM system uses a plurality ofsub-carrier frequencies which are orthogonal in a mathematical sense sothat the sub-carriers' spectra may overlap without interference due tothe fact they are mutually independent. The orthogonality of OFDMsystems removes the need for guard band frequencies and therebyincreases the spectral efficiency of the system. OFDM has been proposedand adopted for many wireless systems. It is currently used inAsymmetric Digital Subscriber Line (ADSL) connections, in some wirelessLAN applications (such as WiFi devices based on the IEEE802.11a/gstandard), and in wireless MAN applications such as WiMAX (based on theIEEE 802.16 standard). OFDM is often used in conjunction with channelcoding, an error correction technique, to create coded orthogonal FDM orCOFDM. COFDM is now widely used in digital telecommunications systems toimprove the performance of an OFDM based system in a multipathenvironment where variations in the channel distortion can be seenacross both subcarriers in the frequency domain and symbols in the timedomain. The system has found use in video and audio broadcasting, suchas DVB and DAB, as well as certain types of computer networkingtechnology.

In an OFDM system, a block of N modulated parallel data source signalsis mapped to N orthogonal parallel sub-carriers by using an InverseDiscrete or Fast Fourier Transform algorithm (IDFT/IFFT) to form asignal known as an “OFDM symbol” in the time domain at the transmitter.Thus, an “OFDM symbol” is the composite signal of all N sub-carriersignals. An OFDM symbol can be represented mathematically as:

$\begin{matrix}{{{x(t)} = {\frac{1}{\sqrt{N}}{\sum\limits_{n = 0}^{N - 1}{c_{n} \cdot {\mathbb{e}}^{{j2\pi}\; n\;\Delta\; f\; t}}}}},{0 \leq t \leq T_{s}}} & (1)\end{matrix}$where Δf is the sub-carrier separation in Hz, Ts=1/Δf is symbol timeinterval in seconds, and c_(n) are the modulated source signals. Thesub-carrier vector in (1) onto which each of the source signals ismodulated cεC_(n), c=(c₀, c₁ . . . c_(N-1)) is a vector of Nconstellation symbols from a finite constellation. At the receiver, thereceived time-domain signal is transformed back to frequency domain byapplying Discrete Fourier Transform (DFT) or Fast Fourier Transform(FFT) algorithm.

OFDMA (Orthogonal Frequency Division Multiple Access) is a multipleaccess variant of OFDM. It works by assigning a subset of sub-carriers,to an individual user. This allows simultaneous transmission fromseveral users leading to better spectral efficiency. However, there isstill the issue of allowing bi-directional communication, that is, inthe uplink and download directions, without interference.

In order to enable bi-directional communication between two nodes, twowell known different approaches exist for duplexing the two (forward ordownload and reverse or uplink) communication links to overcome thephysical limitation that a device cannot simultaneously transmit andreceive on the same resource medium. The first, frequency divisionduplexing (FDD), involves operating the two links simultaneously but ondifferent frequency bands by subdividing the transmission medium intotwo distinct bands, one for forward link and the other for reverse linkcommunications. The second, time division duplexing (TDD), involvesoperating the two links on the same frequency band, but subdividing theaccess to the medium in time so that only the forward or the reverselink will be utilizing the medium at any one point in time. Bothapproaches (TDD & FDD) have their relative merits and are both well usedtechniques for single hop wired and wireless communication systems. Forexample the IEEE802.16 standard incorporates both an FDD and TDD mode.

As an example, FIG. 21 illustrates the single hop TDD frame structureused in the OFDMA physical layer mode of the IEEE802.16 standard(WiMAX).

Each frame is divided into DL and UL subframes, each being a discretetransmission interval. They are separated by Transmit/Receive andReceive/Transmit Transition Guard interval (TTG and RTG respectively).Each DL subframe starts with a preamble followed by the Frame ControlHeader (FCH), the DL-MAP, and the UL-MAP.

The FCH contains the DL Frame Prefix (DLFP) to specify the burst profileand the length of the DL-MAP. The DLFP is a data structure transmittedat the beginning of each frame and contains information regarding thecurrent frame; it is mapped to the FCH.

Simultaneous DL allocations can be broadcast, multicast and unicast andthey can also include an allocation for another BS rather than a servingBS. Simultaneous ULs can be data allocations and ranging or bandwidthrequests.

SUMMARY OF EXAMPLE EMBODIMENTS

In accordance with one embodiment of the present invention, a timingadjustment method for use in a multi-hop communication system isprovided. The system includes a source apparatus, a destinationapparatus and one or more intermediate apparatuses, said sourceapparatus being operable to transmit information along a series of linksforming a communication path extending from the source apparatus to thedestination apparatus via the or each intermediate apparatus, and the oreach intermediate apparatus being operable to receive information from aprevious apparatus along the path and to transmit the receivedinformation to a subsequent apparatus along the path, the system beingconfigured to transmit information in a plurality of consecutivelabelled intervals. The method includes transmitting information in afirst labelled interval from a first said apparatus along a plurality ofconsecutive links of said path via one or more said intermediateapparatuses to a second said apparatus, said information including areference to a particular one of said labelled intervals and thetransmission of that information incurring a delay such that thetransmitted information, or information derived therefrom, is receivedby said second apparatus in a second labelled interval a number of suchintervals after the first labelled interval. The method also includesadjusting said reference to form an adjusted reference referring to afurther labelled interval the or another number of such intervals beforeor after the particular labelled interval.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of particular embodiments of thepresent invention and its advantages, reference is now made to thefollowing description, taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 shows an example of sleep mode in WiMAX;

FIG. 2 shows the RS only relays uplink information for MS;

FIG. 3 shows if an RS does not have timing knowledge of a sleep-mode MS,then it cannot allocate resources on right time for relaying;

FIG. 4 shows the algorithm in RS to solve the problem introduced by theasymmetric links between uplink and downlink;

FIG. 5 shows if an RS cannot obtain enough control information foruplink, the BS may transmit a dedicated message to this RS to inform thescheduled events in uplink;

FIG. 6 shows an MS requests to start sleep mode;

FIG. 7 shows a message flow chart when a BS wants to start sleep mode(BS requests to start sleep mode);

FIG. 8 shows a message flow chart when a BS wants to start sleep mode(RS asks BS to put an MS into sleep mode);

FIG. 9 shows a message flow chart, when an MS in sleep mode (an RS shallallocate bandwidth to MS on time for the scheduled transmission in MSs);

FIG. 10 shows BS sending MOB_SLP-RSP to both RS and MS to stop the sleepmode;

FIG. 11 shows an RS can request to stop the sleep mode in an MS;

FIG. 12 shows an MS can request to stop the sleep mode;

FIG. 13 shows an RS relays both the uplink and downlink;

FIG. 14 shows an MS loses the synchronization with BS because the RSintroduces one-frame delay;

FIG. 15 shows an MS loses the synchronization with BS because the RSintroduces one-frame delay (when RS will delay timing-related controlmessages, it shall modify the messages to compensate the delay);

FIG. 16 shows RS shall modify the timing information to make sure thatthe MS can receive the information during listening windows;

FIG. 17 shows an example that the start time of the first listen windowin MS 2# does not aligned with MS 1#;

FIG. 18 shows the listening window in RS can be shortened by aligningthe start time of the first listening window in MS 2#;

FIG. 19 shows a single-cell two-hop wireless communication system;

FIG. 20 shows applications of relay stations; and

FIG. 21 shows a single hop TDD frame structure used in the OFDMAphysical layer mode of the IEEE 802.16 standard.

DETAILED DESCRIPTION

With signaling in a multi-hop environment there may be some issues withdelays, possibly causing frame latency.

For example, a sleep-mode mobile station (MS) will switch its status tosleep or awake, when falling into sleep window or listening windowrespectively. In other words, within sleep window, base station (BS) andrelay station (RS) cannot transmit messages to MS, while withinlistening window they can. Normally, BS controls the types of sleepmode, and the timing information of each window.

As shown in FIG. 1, a BS will schedule listening, and sleep windows forMS. During listening window, the BS will exchange information with MS.Especially, the BS will transmit a traffic indication to MS duringlistening window to indicate whether there is buffered packet to MS.Therefore, the MS has to be synchronized with BS to ensure exchangingimportant information during listening windows.

In WiMAX relay systems, the relay station (RS) will relay uplink ordownlink information for mobile stations (MSs) and base station (BS).

If the RS just relays the uplink, then it may have no knowledge aboutthe control messages transmitted by BS to MS (downlink), which affectuplink communication. For example, in WiMAX sleep mode, an RS shallallocate bandwidth for MS on time for a scheduled event, which isinstructed by a downlink message. This downlink message may not bedecoded by RS. Therefore, new mechanisms should be designed to supportRS to obtain the relevant control information sent by downlink messages.

Moreover, if the RS relays both the uplink and downlink, it should makesure the MS can be synchronized with BS, especially for sleep-mode MSs.For instance, when an RS cannot relay the timing control informationwithin the current frame, the RS shall adjust the timing informationtransmitted from BS to MS.

Details of the Preferred Power Saving Method We consider two WiMAX relayscenarios, and propose new algorithms to solve the problems of the sleepmode within these scenarios.

Scenario A: Relay Station Relays Both Downlink and Uplink

As shown in FIG. 2, the relay station (RS) only relays MS's uplink, andthe MS can receive all downlink information from BS directly, whichmeans the uplink and downlink is asymmetric.

Since the RS cannot hear control messages transmitted by BS to MS (indownlink), which will affect the uplink communication between MS and BS,then the RS may not perform proper operations, such as resourceallocation, to meet the relevant demands of those control messages.

For example, in sleep mode, MSs may perform scheduled operations, suchas ranging requests. These operations normally will be instructed by thecontrol message in downlink. If the RS does not decode these messages,it cannot allocate resources to MSs on time, thus the scheduledoperations may failed.

As shown in FIG. 3, if RS does not know any sleep-mode timinginformation in the sleep-mode MS, it cannot allocate relay resources forMS to using uplink on time, thus degrading the uplink communication.

There are two algorithms for RS to solve this problem introduced by theasymmetric links between uplink and downlink, as shown in FIG. 4, andFIG. 5.

In the method illustrated in FIG. 4, firstly, an RS will receive PDUs(Protocol Data Units) from BS. Then, it will extract the MAC messagesfrom the received PDUs, and parse the controlling information, whichwill influence the uplink transmission. Eventually, in terms of theparsed controlling information, the RS shall properly schedules theresources, such as time window, and subchannels (or frequency bands),for MSs transmitting message to BS.

In the method illustrated in FIG. 5, if an RS cannot fully collect allthe controlling information, which will affect the uplink communication,the BS can send a dedicated message to RS to inform the scheduled eventsin uplink.

To clearly explain the implementation of the proposed algorithms insleep mode in WiMAX relaying systems, the message flow charts aredescribed below.

Message Chart for Starting the Sleep Mode

A sleep mode can be requested by MS, RS, or BS. FIG. 6. shows themessage flow chart when an MS wants to start sleep mode.

If an MS wants to start sleep mode, it shall asks RS to relay aMOB_SLP-REQ message to BS to request to entering sleep mode.

The BS can approve or refuse the request by sending a message,MOB_SLP-RSP. The BS also needs to ensure the RS can receive thecorresponding information in this MOB_SLP-RSP message within the samesubframe by sending a dedicated message to RS or allowing RS to receivethe sleep-mode control messages.

The message, MOB_SLP-RSP, will indicate the timing information of sleepand listening windows, such as start frame number for first sleepwindow, size information of the listening window, and sleep window.These parameters should also be recorded by RS.

If MS cannot hear any response message from BS within a fixed period, itmeans the request is failed. Then, if necessary, the MS will restart tosend MOB_SLP-REQ to RS.

In lights of the received messages, the RS can schedule the resourcesfor uplink relaying, or it may stop to allocate bandwidth to thecorresponding MS.

The sleep mode also can be requested by BS. FIG. 7. shows the messageflow chart when a BS wants to start sleep mode.

The sleep mode also can be requested by RS. For example, when an RS hasnot enough bandwidth resources for its MSs, it may put some MSs withlower QoS demands into sleep mode. FIG. 8. shows the message flow chartwhen a BS wants to start sleep mode.

Maintaining the Sleep Mode

During the listening window, the BS will transmit traffic indication toMS, and MS shall be awake to receive the necessary information withindownlink subframe. The BS also needs to send the traffic indication andother controlling information relating to this MS to the correspondingRS in the same subframe, thus the RS can predict when the MS will fallin sleep, and can avoid communicating with the MS when it is sleeping.

If an MS receives a traffic indication, which indicates that BS hasbuffered traffic, the MS will keep awake to receive the information fromBS until receiving an MOB_SLP-REQ message from BS again. In terms of theinformation within the received MOB_SLP-REQ, the MS will fall into sleepmode again or terminate the sleep mode.

During the listening window, the RS may allocate bandwidth for MS tomaintaining the connections with active Power-Saving Class.

FIG. 9. shows the message flow chart, when an MS in sleep mode.

The corresponding message flow chart is shown in FIG. 5.

Terminating the Sleep Mode

The sleep mode can be terminated by BS, RS, or MS.

If BS wants to terminate the sleep mode, it has to send a MOB_SLP-RSP toMS and RS within a listen window to tell them when the sleep mode shouldbe ceased. The message flow chart is shown in FIG. 10.

If an RS wants to stop a sleep mode for an MS, it will send anMOB_SLP-REQ message to BS to request to stop this MS's sleep mode. Thenthe BS will send the MOB_SLP-RSP message to both the RS and MS to stopthe MS's sleep mode, or to refuse to cease the sleep mode. Thecorresponding message flow chart is shown in FIG. 11.

If the MS wants to stop the sleep mode, it firstly needs to send anMOB_SLP-REQ message to BS through the RS. Then, the BS will send anMOB_SLP-RSP message to both RS and MS to either stop the sleep mode, orrefuse to stop it. The message flow chart is shown in FIG. 12.

Scenario B: Relay Station Only Relays Uplink for Ms

In this scenario (FIG. 13), the RS relays both the uplink and downlink.

If the RS cannot relay the control messages, which are enclosing timinginformation, to MSs within the same frame (For example, the RS may nothave enough resources to relay the control messages to MSs withincurrent frame), then these control information will be delayed by atleast one frame. In this case, the absolute timing information withinthese timing-related messages for MSs shall be shifted by thecorresponding latency introduced by relaying. If the communication isbi-directional, the RS also needs to inform BS the modified the timinginformation, thus guaranteeing the uplink communication as well.

For example, as shown in FIG. 14, a BS informs the MS to enter sleepmode and start a listening window after two frames, which is in theframe n+3#. Since the RS introduces one-frame delay, the MS actuallystarts the listening window in the frame n+4#, thus losing thesynchronization with BS.

FIG. 15 illustrates the proposed algorithm to solve this problem, whenRS relays timing-related control message, such as MOB_SLP-RSP, andRNG_RSP messages in WiMAX. Firstly, an RS will receive PDUs (ProtocolData Units) from BS. Then, it will extract the MAC messages from thereceived PDUs, and parse the controlling information, which willinfluence the timing control in MSs. If the RS cannot relay atiming-related message within current frame, and the timing informationis absolute, it will modify the timing information in this message, andrelay the message to MSs to compensate its delay. If the communicationis bi-directional, the RS also needs to inform the change of timinginformation to BS as well.

If the system allows RS to enter sleep mode, the proposed algorithm canalso be used to maximize the sleep duration for a RS. For example, whena BS informs an MS to enter sleep mode, the RS can modify the MS's starttime of the first listening window to align the existed sleep-mode MS'sstart time of listening window, thus the possible listening window in RScan be decreased. FIG. 17 shows an example that the start time of thefirst listen window in MS 2# does not aligned with MS 1#. FIG. 18 showsthe listening window in RS can be shortened by aligning the start timeof the first listening window in MS 2#.

Main Benefits

The benefits from particular embodiments may include:

-   -   The proposed method gives an effective approach to support sleep        mode in mobile station in WiMAX relaying systems;    -   The proposed method can have full compatibility with the        IEEE802.16e standard;    -   When an RS just relays uplink traffic, the proposed method can        ensure the RS has the knowledge of the scheduled uplink events,        thus guaranteeing a stable uplink communication;    -   When an RS, which relays both uplink and downlink, will delay        timing-related control messages from BS to MS, the proposed        method can ensure the MS obtaining correct timing information,        thus keeping synchronization between MS and BS;    -   The method can allow RS to request sleep mode for MSs;

Embodiments of the present invention may be implemented in hardware, oras software modules running on one or more processors, or on acombination thereof. That is, those skilled in the art will appreciatethat a microprocessor or digital signal processor (DSP) may be used inpractice to implement some or all of the functionality of a transmitterembodying the present invention. The invention may also be embodied asone or more device or apparatus programs (e.g. computer programs andcomputer program products) for carrying out part or all of any of themethods described herein. Such programs embodying the present inventionmay be stored on computer-readable media, or could, for example, be inthe form of one or more signals. Such signals may be data signalsdownloadable from an Internet website, or provided on a carrier signal,or in any other form.

What is claimed is:
 1. A timing adjustment method for use in a multi-hopcommunication system, the system comprising a source apparatus, adestination apparatus and one or more intermediate apparatuses, saidsource apparatus being operable to transmit information along a seriesof links forming a communication path extending from the sourceapparatus to the destination apparatus via the or each intermediateapparatus, and the or each intermediate apparatus being operable toreceive information from a previous apparatus along the path and totransmit the received information to a subsequent apparatus along thepath, the system being configured to transmit information in a pluralityof consecutive labelled intervals, the method comprising: transmittinginformation in a first labelled interval from a first apparatus along aplurality of consecutive links of said path via one or more saidintermediate apparatuses to a second apparatus, said first apparatusbeing said source apparatus or one of said one or more intermediateapparatuses, said second apparatus being one of said one or moreintermediate apparatuses or said destination apparatus, said informationincluding a reference to a particular labelled interval from among saidplurality of consecutive labelled intervals, and the transmission ofthat information incurring a delay such that the transmittedinformation, or information derived therefrom, is received by saidsecond apparatus in a second labelled interval a number of suchintervals after the first labelled interval; and adjusting saidreference to form an adjusted reference referring to a further labelledinterval from among said plurality of consecutive labelled intervals,the further labelled interval being the or another number of suchintervals before or after the particular labelled interval.
 2. Themethod according to claim 1, wherein said further labelled interval isthe same number of intervals before or after the particular interval assaid number of intervals by which said second labelled interval is aftersaid first interval.
 3. The method according to claim 1, wherein saidconsecutive labelled intervals are consecutively numbered intervals, andwherein said reference and adjusted reference refer to respectivenumbered intervals.
 4. The method according to claim 1, comprisingcarrying out said adjustment in the or one of the intermediateapparatuses between said first and second apparatuses or in said firstapparatus or in said second apparatus.
 5. The method according to claim1, wherein: said further labelled interval is after said particularinterval; said particular labelled interval is the interval in whichsaid second apparatus would receive further information from the firstapparatus if there were no such delay in transmissions from the firstapparatus to the second apparatus; and said further labelled interval isthe interval in which said second apparatus should expect to receivethat further information from the first apparatus given said delay. 6.The method according to claim 1, wherein: said second apparatus isoperable to transmit further information to said first apparatus via theor each intermediate apparatus therebetween in response to theinformation received from the first apparatus; said further labelledinterval is before said particular interval; said particular labelledinterval is the interval in which said first apparatus requires thefurther information from the second apparatus; and said further labelledinterval is the interval in which said second apparatus should transmitthat further information so that the first apparatus receives thatfurther information, or information derived therefrom, in saidparticular labelled interval.
 7. The method according to claim 1,wherein said delay is an expected delay, the method comprising carryingout said adjustment prior to said transmission of said information. 8.The method according to claim 1, comprising carrying out said adjustmentduring said transmission of said information or after said transmissionof said information.
 9. The method according to claim 1, wherein saidfirst apparatus is said source apparatus or a base station; and whereinsaid second apparatus is said destination apparatus or a mobileterminal.
 10. The method according to claim 1, wherein said firstapparatus and/or said second apparatus is an intermediate apparatus. 11.The method according to claim 1, wherein the or each intermediateapparatus is a relay station.
 12. The method according to claim 1,wherein said system is a wireless communication system.
 13. The methodaccording to claim 1, wherein said system is an OFDM or OFDMA system.14. The method according to claim 1, wherein the or each said intervalis an uplink or downlink subframe period of a time division duplexframe.
 15. A multi-hop communication system, comprising: a sourceapparatus, a destination apparatus and one or more intermediateapparatuses, said source apparatus being operable to transmitinformation along a series of links forming a communication pathextending from the source apparatus to the destination apparatus via theor each intermediate apparatus, and the or each intermediate apparatusbeing operable to receive information from a previous apparatus alongthe path and to transmit the received information to a subsequentapparatus along the path, the system being configured to transmitinformation in a plurality of consecutive labelled intervals;transmitting means operable to transmit information in a first labelledinterval from a first apparatus along a plurality of consecutive linksof said path via one or more said intermediate apparatuses to a secondapparatus, said first apparatus being said source apparatus or one ofsaid one or more intermediate apparatuses, said second apparatus beingone of said one or more intermediate apparatuses or said destinationapparatus, said information including a reference to a particularlabelled interval from among said plurality of consecutive labelledintervals, and the transmission of that information incurring a delaysuch that the transmitted information, or information derived therefrom,is received by said second apparatus in a second labelled interval anumber of such intervals after the first labelled interval; andadjustment means operable to adjust said reference to form an adjustedreference referring to a further labelled interval from among saidplurality of consecutive labelled intervals, the further labelledinterval being the or another number of such intervals before or afterthe particular labelled interval.
 16. A non-transitory computer-readablemedium storing a computer program which, when executed on a computingdevice of a multi-hop communication system, causes the system to carryout a timing adjustment method, the system comprising a sourceapparatus, a destination apparatus and one or more intermediateapparatuses, said source apparatus being operable to transmitinformation along a series of links forming a communication pathextending from the source apparatus to the destination apparatus via theor each intermediate apparatus, and the or each intermediate apparatusbeing operable to receive information from a previous apparatus alongthe path and to transmit the received information to a subsequentapparatus along the path, the system being configured to transmitinformation in a plurality of consecutive labelled intervals, the methodcomprising: transmitting information in a first labelled interval from afirst apparatus along a plurality of consecutive links of said path viaone or more said intermediate apparatuses to a second apparatus, saidfirst apparatus being said source apparatus or one of said one or moreintermediate apparatuses, said second apparatus being one of said one ormore intermediate apparatuses or said destination apparatus, saidinformation including a reference to a particular labelled interval fromamong said plurality of consecutive labelled intervals, and thetransmission of that information incurring a delay such that thetransmitted information, or information derived therefrom, is receivedby said second apparatus in a second labelled interval a number of suchintervals after the first labelled interval; and adjusting saidreference to form an adjusted reference referring to a further labelledinterval from among said plurality of consecutive labelled intervals,the further labelled interval being the or another number of suchintervals before or after the particular labelled interval.